This is now the second time Aiyuk and Williams have placed on the list in the past few weeks.
The COVID-19 list was created this summer for a player who either tests positive for or who has been quarantined after having been in close contact with an infected person or persons.
Players will remain on the list until they are medically cleared to return.
Williams, 32, is a former first-round pick of the Redskins back in 2010. He currently in the final year of his five-year, $66 million contract that includes $43.25 million guaranteed and stands to make a base salary of $12.5 million for the 2020 season.
After San Francisco traded for Williams, the expectation is that the two sides will allow the 2020 season to play out before evaluating a potential extension.
In 2020, Williams has started eight games for the 49ers at left tackle.